Bulletin of the American Physical Society
67th Annual Meeting of the APS Division of Fluid Dynamics
Volume 59, Number 20
Sunday–Tuesday, November 23–25, 2014; San Francisco, California
Session R22: Instability: Interfacial and Thin Films III |
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Chair: Brandon Morgan, Lawrence Livermore National Laboratory Room: 2012 |
Tuesday, November 25, 2014 1:05PM - 1:18PM |
R22.00001: Experimental confirmation of multiple base states in two surface thermocapillary driven flows Ranga Narayanan, Brad Messmer, Ichiro Ueno, Thomas Lemee When a temperature gradient is applied along a fluid surface, there will necessarily be a resulting flow along the interface of that fluid. This interfacial flow can result in a bulk movement of the fluid and, at high enough temperature gradients, can cause instabilities and pattern formation. In the present work, experiments are performed on a double free-surface film in a rectangular geometry under thermo-capillary forcing. The films show two basic flow structures at low imposed temperature gradients. In cellular flow, the fluid leaves the hot source toward the cold wall and, upon reaching the cold wall, returns along the sides in a cellular-like structure. For the case of sheet flow, the fluid leaves the hot boundary, moves as a sheet toward the cool wall and then dips into the fluid returning to the hot source along the interior of the film. Simple scaling arguments show that differences arise on account of the lower velocity gradient in the direction of the imposed temperature gradient. This difference in velocity gradients results from inevitable surface deformations. For thin films the effect of surface deformations are magnified because of a larger interfacial-surface-area to volume ratio, whereas thicker films will show less sensitivity to deformations. [Preview Abstract] |
Tuesday, November 25, 2014 1:18PM - 1:31PM |
R22.00002: Spontaneous emulsification dynamics of natural oils Swaminathan Kalimuthu, Mahesh Panchagnula The interface between natural oils and water is partially diffused, which gives rise to interesting spreading behaviors originating from solutal Marangoni instability. This motivates the current experimental study of the resulting spontaneous emulsification of natural oil films. A drop of natural oil is placed on a water surface and the spreading and dewetting dynamics are characterized using high speed imaging. This process involves the nucleation and growth of holes in the oil film. These holes tend to ripen through the coalescence route until the film is emulsified into drops. The spreading and dewetting process can be divided into three regimes viz.,(i) spreading and hole nucleation (dewetting), (ii) growth of holes and (iii) hole ripening (due to coalescence). The process is characterized in terms of the rate of hole nucleation, the surface area fraction (which is a measure of the interfacial energy) as well as the total triple line length separating the two phases. The triple line length is observed to show a power law behavior in time during the first and third regimes with the exponents being 1.5 and 0.3. Finally, the hole nucleation kinetics are shown to follow JMAK model in certain cases while showing a deviation in other cases. [Preview Abstract] |
Tuesday, November 25, 2014 1:31PM - 1:44PM |
R22.00003: Suppression of van der Waals-driven rupture of a bubble moving in a channel Naima Hammoud, Philippe H. Trinh, Howard A. Stone Recent experimental work by Chen et al. (Appl. Phys. Lett. 103, 2013) revealed that when an oil drop is passed through a water-filled channel with hydrophobic walls, rupturing may occur with the droplet adhering to the channel walls if the flow is sufficiently slow. However, if the speed of the flow is increased beyond some critical value, rupturing is suppressed. A scaling argument can be developed to predict this critical transition, but also of interest are the dynamics that govern the transition between stability and instability. In this talk, we shall present further results on this phenomenon through a thin-film model of bubble motion that incorporates the effects of viscosity, surface tension, and van der Waals forces. [Preview Abstract] |
Tuesday, November 25, 2014 1:44PM - 1:57PM |
R22.00004: Influence of interfacial rheology on stabilization of the tear film M. Saad Bhamla, Gerald G. Fuller The tear film that protecting the ocular surface is a complex, thin film comprised of a collection of proteins and lipids that come together to provide a number of important functions. Of particular interest in this presentation is meibum, an insoluble layer that is spread from glands lining our eyelids. Past work has focussed on the role of this layer in reducing evaporation, although conflicting evidence on its ability to reduce evaporative loss has been published. We present here the beneficial effects that are derived through the interfacial viscoelasticity of the meibomian lipid film. This is a duplex film is comprised of a rich mixture of phospholipids, long chain fatty esters, and cholesterol esters. Using interfacial rheology measurements, meibum has been shown to be highly viscoelastic. By measuring the drainage and dewetting dynamics of thin aqueous films from hemispherical surfaces where those films are laden with insoluble layers of lipids at controlled surface pressure, we offer evidence that these layers strongly stabilize the films because of their ability to support surface shearing stresses. This alternative view of the role of meibum can help explain the origin of meibomian gland dysfunction, or dry eye disease, where improper compositions of this lipid mixture do not offer the proper mechanical resistance to breakage and dewetting of the tear film. [Preview Abstract] |
Tuesday, November 25, 2014 1:57PM - 2:10PM |
R22.00005: Coupling between short- and long-wavelength modes of interfacial instabilities Iman Nejati, Mathias Dietzel, Steffen Hardt The principles and behavior of (hydrodynamic) self-organizing systems (SOS) are well understood, whereas comparably few studies have addressed the coupling of such systems. Given that in a SOS the cooperative effects of sub-units lead to the coherent behavior of the assembly (commonly revealed by the spontaneous generation of spatial and/or temporal patterns), interactions between individual SOS may give rise to collective behavior as well. Within this context an experimental analysis of two conjugated stacked liquid layers has been performed. The lower film is placed on a heated plate and is much thinner (less than 10 microns) than the upper one (a few hundred microns), which is situated under a cooled air layer. The films evolve according to coupled short- and long-wavelength (SW/LW) modes of interfacial instabilities. The liquid-liquid (L-L) interface is deformed by viscous stresses with the same wavelength as the hexagonal pattern emerging due to the SW-B\'{e}nard-Marangoni instability at the liquid-gas interface of the thicker layer. For gravity-inverted systems, the pattern symmetry is affected by the LW-Rayleigh-Taylor instability developing at the L-L interface, at least if the lower film is sufficiently thick (avoiding the stabilization by the disjoining pressure). [Preview Abstract] |
Tuesday, November 25, 2014 2:10PM - 2:23PM |
R22.00006: Imposing periodic suction to stabilise thin-film flow down an inclined plane Alice Thompson, Demetrios Papageorgiou, Dmitri Tseluiko Flow of a thin film down an inclined plane becomes unstable when the slope angle or Reynolds number are sufficiently large; enhancement or suppression of these instabilities is relevant to a range of industrial applications. Here we study the effect of introducing spatially periodic blowing and sucking through the rigid planar boundary. We derive two long-wave, thin-film models to describe the system, including the imposed suction as well as inertia, surface tension, gravity and viscosity. We explore the bifurcation structure in each model, and perform linear stability and time-dependent simulations for both small and large forcing amplitude. Both models predict that forcing via imposed suction can be chosen to either destabilize or stabilize the flow, and we show that forcing at very long wavelengths always has a stabilizing effect on the flow. [Preview Abstract] |
Tuesday, November 25, 2014 2:23PM - 2:36PM |
R22.00007: Thermocapilary transport in thin films using traveling thermal waves Valeri Frumkin, Alex Oron, Wenbin Mao, Alexander Alexeev We use modeling and direct numerical simulations to investigate the nonlinear dynamics of a two-layer system consisting of a thin liquid film and an overlying gas layer sandwiched between two solid walls. Fluid flow in the system is driven by the Marangoni instability induced by thermal waves propagating along the bottom wall. We show that for relatively small Marangoni numbers interfacial capillary waves form in the thin film that transport liquid along the solid wall. In this case, the frequency of thermal waves leading to the most efficient net transport is defined by their wave length and weakly depends on other system parameters. For larger Marangoni numbers which are still sufficiently small to prevent film rupture, a periodic structure consisting of localized drops interconnected by thin liquid bridges emerges. This train of drops travels unidirectionally along the heated substrate following the thermal wave and effectively transport liquid enclosed in the drops. The results of our study are useful for developing new approaches for transporting and directing liquids in microfluidic systems with a free surface. [Preview Abstract] |
Tuesday, November 25, 2014 2:36PM - 2:49PM |
R22.00008: Pulse interaction in non-local active-dissipative systems modeling falling liquid films with external effects Dmitri Tseluiko, Te-Sheng Lin, Marc Pradas, Serafim Kalliadasis, Demetrios Papageorgiou, Mark Blyth We analyze pulse interaction in active-dissipative systems that arise in the study of falling liquid films in the presence of various external effects, e.g. an applied electric field or turbulent gas flow. Such effects result in additional non-local terms in the form of pseudo-differential operators. We analyze both weakly nonlinear and fully nonlinear reduced model equations. Our analysis is an appropriate extension of our previous studies of pulse interaction for local equations, both weakly nonlinear and model equations [1,2], to non-local ones. We compare the theoretical predictions with numerical results for reduced model equations and Stokes flow. It is found that non-locality strongly influences pulse interactions and results in several features that are not present in local equations. \\[4pt] [1] D. Tseluiko and S. Kalliadasis 2014 ``Weak interaction of solitary pulses in active dispersive-dissipative nonlinear media,'' IMA J. Appl. Math. {\bf 79} 274-299. \\[0pt] [2] M. Pradas, D. Tseluiko and S. Kalliadasis 2011 ``Rigorous coherent-structure theory for falling liquid films: Viscous dispersion effects on bound-state formation and self-organization,'' Phys. Fluids {\bf 23} 044104. [Preview Abstract] |
Tuesday, November 25, 2014 2:49PM - 3:02PM |
R22.00009: Partial liquid-penetration inside a deep trench by film flowing over it Phuc-Khanh Nguyen, Yiannis Dimakopoulos, John Tsamopoulos Liquid film flow along substrates featuring a deep trench may not wet the trench floor, but create a second gas-liquid interface inside the trench. The liquid penetration inside the trench depends on the location and shape of this inner interface. The penetration increases by decreasing the two three-phase contact lines between the inner interface and the two side-walls or the flow rate and depends on the liquid properties. This partial-penetration is studied by employing the Galerkin / finite element method to solve the two-dimensional steady-state Navier-Stokes equations in a physical domain that is adaptively remeshed. Multiple branches of steady solutions connected via turning points are revealed by pseudo arc-length continuation. Flow hysteresis may occur in a certain range of liquid penetration depth, when the interaction of the two interfaces changes qualitatively. This induces an abrupt jump of penetration distance and deformation amplitude of the outer interface. [Preview Abstract] |
Tuesday, November 25, 2014 3:02PM - 3:15PM |
R22.00010: Can numerical simulations accurately predict hydrodynamic instabilities in liquid films? Fabian Denner, Alexandros Charogiannis, Marc Pradas, Berend G.M. van Wachem, Christos N. Markides, Serafim Kalliadasis Understanding the dynamics of hydrodynamic instabilities in liquid film flows is an active field of research in fluid dynamics and non-linear science in general. Numerical simulations offer a powerful tool to study hydrodynamic instabilities in film flows and can provide deep insights into the underlying physical phenomena. However, the direct comparison of numerical results and experimental results is often hampered by several reasons. For instance, in numerical simulations the interface representation is problematic and the governing equations and boundary conditions may be oversimplified, whereas in experiments it is often difficult to extract accurate information on the fluid and its behavior, e.g. determine the fluid properties when the liquid contains particles for PIV measurements. In this contribution we present the latest results of our on-going, extensive study on hydrodynamic instabilities in liquid film flows, which includes direct numerical simulations, low-dimensional modelling as well as experiments. The major focus is on wave regimes, wave height and wave celerity as a function of Reynolds number and forcing frequency of a falling liquid film. Specific attention is paid to the differences in numerical and experimental results and the reasons for these differences. [Preview Abstract] |
Tuesday, November 25, 2014 3:15PM - 3:28PM |
R22.00011: Instabilities of structured metal films on nanoscale Nanyi Dong, Yueying Wu, Jason Fowlkes, Philip Rack, Lou Kondic We consider instabilities of metal films on nanoscale, with particular focus on the interplay between the initial geometry and instability development. In experiments, metal films are deposited lithographically, allowing for precise control of the initial shape, and then exposed to laser pulses that liquefy them. The considered geometries involve various shapes (cylinders or prisms) superimposed on top of a flat film. We consider this problem within the framework of the long wave (lubrication) theory. Our simulations show that the main features of the instability development could be captured, as long as destabilizing liquid-solid interaction is considered in the model. We conclude by discussing the influence of the distance between the imposed perturbations, their shape, as well as experimental noise on the evolution. [Preview Abstract] |
Tuesday, November 25, 2014 3:28PM - 3:41PM |
R22.00012: Viscous ferrofluid films under the effects of magnetic fields Devin Conroy, Alex Wray, Omar Matar We consider a thin, ferrofluidic film flowing down an inclined substrate, under the action of a magnetic field, bounded above by an inviscid gas. The fluid is assumed to be weakly-conducting. Its dynamics are governed by a coupled system of the steady Maxwell's, the Navier-Stokes, and the continuity equation. The magnetisation of the film is a function of the magnetic field, and may be prescribed by a Langevin function. We make use of a long-wave reduction in order to solve for the dynamics of the pressure and velocity fields inside the film. The potential in the gas phase is solved with the use of Fourier Transforms. Imposition of appropriate interfacial conditions allows for the construction of an evolution equation for the interfacial shape via use of the kinematic condition. The magnetic effects give rise to a non-local contribution. We conduct a parametric study of both the linear and nonlinear stability of the system in order to evaluate the effects of the magnetic field. [Preview Abstract] |
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